Absorbent cores are used in absorbent products, such as baby diapers, incontinence products and sanitary towels. Commonly, such absorbent cores include cellulosic fibers and/or superabsorbents. The absorbent cores can be formed individually or they can be formed as a web, which afterwards is divided into individual absorbent cores.
A typical production line includes one or more mat-formers, which form an absorbent material, being in the form of individual products or as a web, by air-lying. EP 1,253,231A2 discloses a method for forming a fiber web by means of air-lying fibers via a number of mat-formers, but a production line could also be run with only one mat-former.
There is a constant desire to run production lines for absorbent cores faster, in order to speed up the production. However, when running faster, the material formed in the mat-forming step tends to become more uneven, i.e. the variation in weight per unit area increases.
There is also a trend to make thin products, since these will take up less space and thereby be cheaper to package, transport and store, as well as being more convenient for the user. Therefore, it is common to compress the absorbent material between compression rolls to reduce its thickness.
However, it has been noticed that, when compressing a somewhat uneven material to relatively highly reduced thickness, so called hard-spots might appear. Hard spots may be described as local regions in the core being more compact and stiffer than the overall core. It is believed that the hard-spots form from the compression of regions having higher basis weight than the surrounding regions after the mat-forming step. Hence, hard-spots may appear in particular when using relatively high production speeds and for relatively thin absorbent cores.
Hard-spots can be felt by a user of the absorbent product both before use (e.g. by touching the products by hand) and in use. Being relatively stiff, the hard-spots may be perceived by a user as causing a less comfortable and less efficient product. This is particularly the case for larger hard-spots. Hence, it is desirable to avoid hard-spots as far as possible.
There is therefore a need for an improved method for forming an absorbent core.